Linear olefins having at least 10 and at most 20 carbon atoms per molecule (hereinafter referred to as "linear C.sub.10 -C.sub.20 olefins") are valuable starting materials for the preparation of synthetic detergents. Said olefins can be obtained by mild thermal cracking of mixtures of linear paraffins having at least 20 carbon atoms per molecule (hereinafter referred to as "linear C.sub.20.sup.+ paraffins"). Linear C.sub.20.sup.+ paraffins, together with branched C.sub.20.sup.+ paraffins are found in heavy mineral oil fractions, such as lubricating oil fractions. The paraffins can be separated from said mineral oil fractions by cooling. From the paraffin mixtures obtained the desired linear paraffins can be isolated by way of fractional crystallization or complexing with urea. The linear C.sub.20.sup.+ paraffins thus obtained are generally contaminated with sulphur- and nitrogen compounds from the mineral oil, as well as cyclic compounds. Preparatory to being suitable for use as feed for the preparation of linear C.sub.10 -C.sub.20 olefins by mild thermal cracking the C.sub.20.sup.+ paraffins should be freed from these contaminants.
Linear C.sub.20.sup.+ paraffins which can suitably be used as starting material for the preparation of linear C.sub.10 -C.sub.20 olefins by way of mild thermal cracking may also be synthesized starting from a mixture of carbon monoxide and hydrogen. In this what is called Fischer-Tropsch synthesis a H.sub.2 /CO mixture is contacted at elevated temperature and pressure with a catalyst comprising one or more metals from the iron group together with one or more promoters and a carrier material. The preparation of these catalysts can suitably be carried out by the known techniques, such as precipitation, impregnation, kneading and melting. As compared with waxy heavy mineral oil fractions the products prepared by the Fischer-Tropsch synthesis have the advantage that they contain virtually no sulphur- and nitrogen compounds and cyclic compounds. Nevertheless the use of the products obtained over the usual Fischer-Tropsch catalysts for the preparation of linear C.sub.10 -C.sub.20 olefins has two drawbacks which are connected with their composition. In the first place, these products contain but a relatively small amount of C.sub.20.sup.+ compounds as compared to the quantities of C.sub.19 - compounds present. Furthermore, the C.sub.20.sup.+ compounds are made up to a considerable extent of branched paraffins, branched and unbranched olefins and oxygen-containing compounds.
Recently there has been found a class of Fischer-Tropsch catalysts which have the property of yielding a product in which considerably more C.sub.20.sup.+ compounds are present than in the products prepared by the usual Fischer-Tropsch catalysts, which C.sub.20.sup.+ compounds consist virtually exclusively of linear paraffins. The Fischer-Tropsch catalysts belonging to the above-mentioned class contain silica, alumina or silica-alumina as carrier materials, and cobalt together with zirconium, titanium and/or chromium as catalytically active metals, in such quantities that per 100 pbw of carrier material the catalysts comprise about 3-60 pbw of cobalt and about 0.1-100 pbw of zirconium, titanium, and/or chromium. The catalysts are prepared by depositing the metals involved on the carrier material by kneading and/or impregnation. For further information concerning the preparation of these catalysts by kneading and/or impregnation reference may be made to Netherlands Patent Application No. 8301922, which is commonly assigned copending application, Ser. No. 594618, filed Mar. 29, 1984, now U.S. Pat. No. 4,522,939, issued June 11, 1985. Considering the composition of the product prepared over the cobalt catalysts, it is extremely attractive to separate from the product a heavy fraction substantially consisting of C.sub.20.sup.+ paraffins and to convert at least part of this heavy fraction by mild thermal cracking into a mixture of hydrocarbons substantially consisting of linear olefins and containing the desired C.sub.10 -C.sub.20 olefins.